Explanation:
At the center of our Milky Way Galaxy lies a
supermassive black hole.
Once a controversial claim, this conclusion is now solidly based
on 16 years of observations
that map the orbits of 28 stars very near
the galactic center.
Using European Southern Observatory
telescopes and sophisticated
near infrared cameras, astronomers patiently measured the positions
of the stars over time, following one star, designated S2, through
a complete orbit as it came within about 1 light-day of the
center of the Milky Way.
Their results
convincingly show that S2 is moving under the influence of the
enormous gravity of a compact, unseen object -- a black hole
with 4 million times the mass
of
the Sun.
Their ability to track stars so close to the
galactic center
accurately measures the black hole's mass and also determines the
distance to the center to be 27,000 light-years.
This deep, near-infrared image shows the crowded inner 3
light-years of the central Milky Way.
Spectacular time-lapse animations of the stars orbiting
within light-days of the galactic center
can be found here.